Zinc oxide is widely used in rubber, UV resistant plastics, cosmetic formulations (including lotions, ointments, creams, antiseptics, etc.), paint, sensors, LED's, transistors and other microelectronic devices, medical devices, etc. Zinc oxide is used in these applications for a wide range of purposes, including as an activating agent, as a UV blocking agent, to reduce the effects of sebum in cosmetic applications, as a pigment, a piezoelectric, an antiseptic agent, for sunscreen protection, as a semiconductor, a transparent conducting oxide, a pryoclectric or dielectric agent, and the like. By itself, zinc oxide is generally not used as a filler due to its expense. For rubber compounding, zinc oxide is used as an activator to initiate the rubber accelerators to begin the vulcanization process. However, most of the zinc oxide is not consumed at vulcanization. Only a small amount at the surface of the zinc oxide is consumed. The remainder of the zinc oxide is, in practical effect, wasted.
New applications of zinc oxide in recent years tend to emphasize small particle size. However, small particle size zinc oxide may be difficult to produce, difficult to-de-agglomerate, and difficult to disperse. The smaller zinc oxide particles can also unduly impair rheology characteristics of a formulation. Fine zinc oxide particles also tend to impair the spreadability and usability of cosmetics and the like.
Accordingly, coating a thin layer of zinc oxide on other, small particle size pigments is proposed as an alternative to using zinc oxide per se. Zinc oxide coated silica, clay, talc, and mica has been suggested to reduce consumption of zinc oxide in rubber compounding. It has also been suggested to coat mica with zinc oxide in order to improve its spreadability.
Zinc oxide also has been used in combination with calcium carbonate to help with the dispersion of zinc oxide in rubber compounding. Attempts include physically mixing zinc oxide with calcium carbonate, and then using the mixture in rubber compounding. This approach appears suitable for only limited situations and/or with marginal effects.
A variety of coated calcium carbonates and sulfates are commercially available. These have included zinc carbonate coated calcium carbonate, zinc oxide coated calcium carbonate, zinc carbonate coated calcium sulfate, and zinc oxide coated calcium sulfate. These generally have been prepared using ground, not precipitated, calcium materials. Analysis of such products suggests that the zinc material is coated on relatively large, rather low surface area calcium carbonate or calcium sulfate. The bulk density of these also appears to be relatively large, e.g., on the order of about 0.6 to 1.1 g/ml. The zinc oxide content of these coated calcium carbonates or sulfates also is believed to range from about 30 to about 55% on a weight basis.
Japanese Patent Application No. JP 2001/0336919, published on Jul. 31, 2002, suggests a method of coating calcium carbonate with a zinc salt of a lower fatty acid or of a mineral acid. The mixture is then thermally baked to decompose a layer of zinc oxide on the surface of the calcium carbonate. Baking of organic materials may release carbon black, which is undesirable for usually white or yellowish zinc oxide. Baking of a mineral salt also may release toxic or highly corrosive materials like sulfur dioxide, hydrogen chloride, etc.
In U.S. Pat. Nos. 4,810,578, 5,840,795, 6,013,699, and 6,147,151, clay, especially clay that is surface-modified with silane coupling agents, is increasingly being used in tires and other rubber compounds. The silanes and clay are used with the intent to increase the modulus and reduce heat build-up of the rubber compound, so that the rolling resistance of the tires decreases, and fuel savings are realized. However, clays alone tend to reduce abrasion resistance in tire formulations as mentioned in U.S. Published Patent Application No. 2004/0249045 by Goodman et al. (Dec. 9, 2004), the relevant portions of which are incorporated herein by reference.
Some rubber compounds, such as those used in shoe soles, rubber bands, etc, need to be transparent or translucent. Unfortunately, ordinary zinc oxide by itself impairs transparency and/or translucency. For example, French process zinc oxide will render a cured rubber compound white and non-transparent at concentrations even as low as 1.5 phr (parts per hundred parts of rubber). Zinc carbonate may be used instead of zinc oxide in rubber compounding at somewhat higher concentrations. However, at 5 phr, good commercial zinc carbonate still does not render the rubber compound totally non-transparent.